Thanks for giving advice patkar_a. Sorry for late reply.
Nowadays My Ertiga Vxi Green manages to give me mileage of minimum 15 km/ltr in City driving. On Highway (Pune - Kolhapur - Pune) it touches at 20 km/ltr with AC ON Fan speed at 2 and 4 Adults and 1 Kid with luggage. All this on Petrol Mode
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Total vehicle run is 7900 kms and serviced just 2 times till date.